1. Field of the Invention
The present invention relates to a negative electrode for lithium-ion secondary battery and a lithium-ion secondary battery.
2. Description of the Related Art
Since lithium-ion secondary batteries are light and have a high capacity in comparison with nickel-cadmium batteries, nickel-hydride batteries, and the like, the lithium-ion secondary batteries are widely used as power sources for portable electronic devices. In addition, the lithium-ion secondary batteries are prime candidates as power sources mounted in hybrid vehicles and electric vehicles. In recent years, along with downsizing and high functionality of portable electronic devices, the lithium-ion secondary batteries with a higher capacity, which are the power sources of these portable electronic devices, are expected.
The capacity of the lithium-ion secondary batteries mainly depends on active materials of electrodes. As for negative electrode active materials, graphite is generally used. However, a theoretical capacity of graphite is 372 mAh/g, and batteries in practical use with a capacity of about 350 mAh/g are already used. Accordingly, in order to obtain nonaqueous electrolyte secondary batteries having sufficient capacity as energy sources of high-functional portable devices in the future, there are needs for achieving a higher capacity, and negative electrode materials having larger theoretical capacity than that of graphite are needed for achieving a higher capacity.
At present, alloy-based negative electrode materials such as silicon and silicon oxide have attracted attention. Silicon can electrochemically intercalate and deintercalate lithium ions and charging and discharging with a very large amount of capacity compared with graphite are possible. In particular, it is known that the theoretical discharge capacity of silicon is about 4210 mAh/g, which is 11 times the theoretical discharge capacity of graphite.
However, when silicon and silicon oxide are used as a negative electrode active material to achieve a high capacity, several problems arise. Specifically, when lithium is intercalated, a very large amount of volume expansion is accompanied due to a change from the original crystal structure by forming a lithium-silicon alloy. For this reason, exfoliation of a negative electrode active material layer or fracture of the negative electrode occurs, and there is a problem of remarkably large cycle deterioration.
In order to solve the above problem due to the expansion, the volume expansion is suppressed by using a negative electrode current collector having high tensile strength such that a 0.2% proof stress is 250 N/mm2 or more or tensile strength is 300 N/mm2 or more to suppress a fracture of the negative electrode, and adhesion between the negative electrode active material layer and the negative electrode current collector are improved by using the negative electrode current collector with a surface roughness Rz of 0.6 to 10 μm by a roughening treatment (for example, Japanese Unexamined Patent Application No. 2011-192563). Moreover, the thickness of the negative electrode active material layer is limited so that the tensile strength of the negative electrode current collector is 3.82 N/mm or more, and the tensile strength of the negative electrode current collector per 1 μm thickness of an active material thin film is 1.12 N/mm or more (for example, Japanese Patent 3733070).